1. Field of the Invention
This invention generally relates to a device for developing an electrostatic latent image formed on an imaging surface using toner particles to obtain a visual toner image, and, in particular, to a device for developing an electrostatic latent image using magnetic toner particles.
2. Description of the Prior Art
In electrophotographic copying and printing machines and facsimile machines or the like, an electrostatic latent image is formed on an imaging surface such as the surface of a photosensitive member, and, then, the thus formed latent image is developed by a developer, normally called toner, to obtain a visual toner image. A device for developing such a latent image using highly electrically resistive, magnetically attractive toner particles is well known in the art. However, this type of toner particles usually do not bear real charges, and, thefore, a bias potential is normally applied to a developing sleeve on which toner particles are carried. Thus when the toner particles are brought closer to the imaging surface on which an electrostatic latent image to be developed is formed, charges are induced in the toner particles under the influence of an electric field created between the developing sleeve and the latent image on the imaging surface whereby the induced charges in the toner particles are used to carry out development, i.e., attraction of toner particles to the latent image.
For this reason, in a prior art device for developing an electrostatic latent image using insulating and magnetic toner particles, requirements for the potential of an electrostatic latent image to be developed and for the bias potential to be applied to a developing sleeve have been rather severe. Another disadvantage in the prior art device is that when charges are induced as described above, electrically unstable toner particles may adhere to those portions of the imaging surface other than image portions, which could cause the so-called background contamination. In order to obviate such disadvantages, a trial has been made to lower the resistivity of toner particles; however, a reduction in resistivity would cause a deterioration in image transfer efficiency thereby bringing about another disadvantage. Thus there has been a need to develop a novel developing device capable of developing a latent image using electrically insulating, magnetic toner particles stably at all times.